Lesson 2 Objectives

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Lesson 2 Objectives

• General overview of neutron physics that you
  already know
• Thinking like a neutron
• What stops a criticality?
• MAGICMERV
• Buckling equivalence

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DOE-STD-1135-99
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DOE-STD-1135-99
1.0 Nuclear Theory The basics of nuclear physics
and nuclear reactor theory are mandatory for
understanding the fundamentals for performing the
function of a criticality safety engineer.
Information below can be obtained through various
tools including appropriate college textbooks.
See Appendix A for available training
resources.   1.1 Fission Process   The individual
should be able to a. Define the following terms
Excitation energy, Cross Section, Fissile
material, Fissionable material, Fertile
material. b. Sketch the fission cross section for
both U-235 and Pu-239 as a function of neutron
energy. Label each significant energy region and
explain the implications of the shape of the
curves for criticality safety. c. Explain why
only the heaviest radioactive nuclei are easily
fissioned. d. Explain why uranium-235 fissions
with thermal neutrons and uranium-238 fissions
only with fast neutrons. e. Characterize the
fission products in terms of mass groupings and
radioactivity. f. Define sub-critical, critical,
super-critical, nu, and beta. g. Define
reactivity and describe how it is measured. h.
Explain the Six-Factor formula and the terms used
therein. i. Explain how delayed neutrons affect
reactivity. j. Explain the effects of the
following factors relevant to criticality safety
of operations Mass, Interaction, Geometry,
Moderation, Reflection, Concentration, Volume,
Neutron absorbers and Enrichment.
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DOE-STD-1135-99
1.2 Various Types of Radiation Interaction with
Matter   The individual should be able to a.
Describe the interactions of the following with
matter Alpha particle, Beta particle, Positron,
and Neutron. b. Describe the following ways that
gamma radiation interacts with matter Compton
scattering, Photoelectric effect, Pair
production   1.3 Neutron Absorbers   The
individual should be able to a. Describe the use
of neutron poisons. b. Explain the absorption
characteristics of the following elements in
terms of their cross-sections cadmium, boron,
chlorine, gadolinium, and hydrogen. c. Explain
the purpose and use of Raschig Rings as a neutron
poison.
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Figure 1 Neutron Absorption in Boron
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Figure 2 Neutron ScatteringModeration
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Figure 3Neutron-Induced Fission
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Figure 4 Summary of Neutron Interactions
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Figure 5 Chain Reactions
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Criticality Neutron balance

• Critical configuration Neutron PRODUCTION from
  fission exactly balances neutron LOSS from
  absorption and leakage

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Criticality Neutron balance (2)

• Our focus is a little different from reactor
  physics because we are much more influenced by
  LEAKAGE
• In this regard, we are much closer to Fermi, et
  al., because of the UNIQUENESS of our situations
  and our strong dependence on SIZE and SHAPE of
  the system being considered

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U-235 Sphere
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What if we fail?

• What stops a criticality accident once it starts?
• Temperature feedback (metals)
• Void formation (solutions)
• Material dispersion (solutions)
• Geometric rearrangement (metals and solutions)
• Big problem recriticality

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Time History of Criticality Accidents
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MAGICMERV

• Simple checklist of conditions that MIGHT result
  in an increase in k-eff.
• Mass
• Absorber loss
• Geometry
• Interaction
• Concentration
• Moderation
• Enrichment
• Reflection
• Volume

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Parameter 1 Mass

• Mass Mass of fissile material in unit
• More is worse -- higher k-eff (usually).
• Possible maximization problem. (Example?)
• Should allow for measurement uncertainties (e.g.,
  add 10 for assay accuracy)
• Parametric studies?

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Figure 7 Effects of Mass on a Fission Chain
Reaction
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Parameter 2 Loss of absorbers

• Loss of absorbers Losing materials specifically
  depended on for crit. control
• More (loss) is worse
• Not usually a problem because not usually used
• We specifically avoid this situation by removing
  all absorbers we can identify (e.g., can walls,
  boron in glass)
• BE CAREFUL Fruitful area for contention
• Parametric studies?

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Figure 8 Nuclear Poison Capture of Thermal
Neutrons
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Parameter 3 Geometry

• Geometric shape of fissile material
• Worst single unit shape is a sphere Lowest
  leakage
• Worst single unit cylindrical H/D ratio 1.00
• 0.94 in a buckling homework problem
• Do not depend on either of these in situations
  with multiple units
• Parametric studies?

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Figure 9 Typical Containers
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Figure 10 Favorable vs. Unfavorable Geometry
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Parameter 4 Interaction

• Interaction Presence of other fissile materials
• More is usually worse. (Counterexample?)
• Typical LATTICE study
• Number
• Arrangement
• Stacking
• Other processes (e.g., material movement) in same
  room
• Hold-up
• Parametric studies?

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Figure 11 Neutron Interaction
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Figure 12 Example of Physical Controls on
Interaction
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Parameters 5 Concentration

• Concentration
• Solution concentration
• Considered in addition to mass, volume,
  moderation because of CONTROL possibilities
• No new physics here

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Parameter 6 Moderation

• Moderation Non-fissile material that is
  intermingled with fissile material
• Slows down the neutrons
• Affects absorption (up) and leakage (down)
• More is usually worse.
• Simultaneously a reflector
• Usual cases
• Other material in vicinity of unit (structure,
  equipt)
• Water from sprinklers
• Operator body parts
• Parametric studies?

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Figure 14 Energy Losses in
Neutron Collisions
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U-235 Cross sections
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Hydrogen total cross section
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Critical mass curve
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Parameter 7 Enrichment

• Enrichment fissile in matrix
• U-235, Pu-239, U-233 (?)
• Higher is worse. (Counterexamples?)
• Source of problem in Tokai-mura accident
• Parametric studies?

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Parameter 8 Reflection

• Reflection Non-fissile material surrounding the
  fissile unit
• Effect of interest Bouncing neutrons back
• More is worse. (Counterexamples?)
• Usual cases
• People 100 water without gap
• Floors
• Walls Assume in corner
• Worse than water Poly, concrete, Be
• Do not underestimate nonhydrogenous reflectn
• Parametric studies?

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Figure 15 Nuclear Reflection
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Parameter 9 Volume

• Volume Size of container holding fissile
  material
• Usually of concern for
• Spacing of arrays (Less is worse.)
• Flooding situations. (More is worse.)
• Very sensitive to fissile mass
• Parametric studies?

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Instapundit link
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Instapundit link